The goal of the proposal is understanding the link between inflammation associated with low-grade bacterial infections and thrombotic processes. The underlying hypothesis is that acute or chronic low-grade infection primes platelets and thrombin-generating pathways to increase propensity for thrombosis. The hypothesis will be approached by measuring responses, in vivo and in vitro, of platelets, the clotting system and the vascular anticoagulant system, to acute and chronic doses of endotoxin from Gram-negative bacteria. The studies will take advantage of mice with genetic deficiency in the phylogenetically conserved receptor (toll-like-receptor-4) required for initiating innate immunity to endotoxin of Gram-negative bacteria. Measurements will include intravascular thrombin generation and inhibition and platelet microaggregation in vivo. In addition a family of in vitro assays of platelet physiology and pharmacology, leukocyte interactions and thrombin generation in blood in conjunction with measurement of inflammatory cytokines will provide information about changes in threshold, gain and amplitude of thrombogenic responses. The unique aspect of this proposal is the ability to distinguish acute, non-genomic actions of endotoxin on platelets through the innate immune response, from longer term consequences of changes in gene transcription. Effects of chronic low grade bacterial infection will be studied in humans undergoing initial therapy for advanced bacterial periodontal disease. Blood samples will be obtained from these patients before and after successful treatment, which does not involve pharmacotherapy. These blood samples will be assayed in parallel with those from the mice. This application is highly responsive to the RFA, because it focuses on translational research at the interface of thrombosis and inflammation. The toll family receptors of innate immunity are well characterized for defense against bacteria. We will extend this characterization by defining the mechanism by which these receptors influence thrombotic propensity.